The present invention generally relates to conveying or feeding sheets or sheet material such as, for example, corrugated blanks, in a box finishing machine. In such machine it is important that the sheets are fed in synchronism with the operations performed at the stations along the machine, such operations being, for example, printing, slotting and scoring, folding and gluing. In the box finishing machine art, synchronous feeding of the sheets relative to the cycle of operation at the various stations along the machine is often referred to as "register feeding" or "feeding in register". In order for the operations such as printing, slotting, scoring, folding and gluing to be performed at the right locations on the sheet, it is obvious that the sheet must arrive at the stations at precisely the right times.
In a box finishing machine, for example, corrugated blanks are fed from a vertical hopper one by one from beneath the hopper by means of a first feeder which sequentially transports the blanks from the bottom of the hopper to a second feeder positioned at the beginning or inlet of the box finishing machine. In conventional machines, the second feeder may be nip rolls or feed rolls. Downstream of the nip or feed rolls, pull rolls are employed to convey the blanks from one station to the next station. Pull rolls include a bottom roll typically made from steel with a smooth surface, and a plurality of collars above the steel roll and rotating in counter direction, the collars typically having a knurled steeled surface or a rubber surface. The sheets are fed between the collars and the underlying steel roll which then proceed to feed the sheet along the path of the finishing machine.
One of the problems which attends the use of such pull rolls is that they can crush or deform the corrugated board if the gap between the collars and the underlying roll is not precisely set. Crushing of the board of course reduces the strength of the ultimate box product. Deformation of or contact with the board, even slight deformation or contact, is detrimental to printing on the board. Another problem which can be experienced with such pull rolls is that they at times do not sufficiently grip the blanks. In addition when the sheets or blanks being fed are warped, the pull rolls are unable to flatten the sheets or blanks and resulting in loss of register or jams in the feeding. Furthermore when the blanks are received from a printing station, the collars can smear the ink that was just printed.
Another type of feeder known in the box-making industry as feed rolls or nip rolls and which has been used in the industry includes an underlying roll typically having a knurled steel surface and an upper roll having for example a steel core and a grooved rubber surface layer. The sheet or corrugated blank being fed is of course gripped between the rolls and fed along the path of the finishing machine. With such, the sheet or blank being fed is still susceptible to crushing or deformation, and furthermore it will not be gripped with sufficient force if the gap between the rollers is not set to precise dimension. Moreover the precise setting of the gap is not predictable with such rolls. Furthermore like the pull rolls described above, the presently described feed rolls also are unable to flatten warped sheets or blanks resulting in loss of register and jamming. In addition, the deformation of the flexible or deformable feed roll surfaces causes variation in surface speed resulting in loss of register and roll wear. In order to prevent injury to an operator's hands, it is necessary to install a nip guard adjacent to the nip rolls. However nip guards can cause a jam or inhibit feeding of warped blanks.
More recently a vacuum type conveyor has been used in which for example a wheel or belt conveyor is contained in a vacuum box so that the vacuum holds the sheet or blank on the belt or wheels of the conveyer. However, the problem with this method is that if the vacuum in the vacuum box is constant, large air losses occur in the spaces between successive sheets or blanks being fed thus requiring a very large volume of vacuum and vacuum source not to mention the noise that attends such installations.
In an attempt to overcome this problem, application of the vacuum is timed with the flow of the sheets or blanks. However this imposes a limitation on the speed of the feeding process and in turn production while further requiring complicated and expensive mechanisms in order to effect the periodic application of vacuum in timed relationship with the flow of sheets or blanks. In addition, with a vacuum system, the amount of vacuum that can be applied to the sheets is limited and thus loss of register can result.